1. Field of the Invention
The present invention relates to insert chips and a manufacturing method thereof. Insert chips are used as blades (teeth) of a tricone bit which is a tool for drilling an oil well (hereinafter referred to as oil-drilling tricone bit). Specifically, the insert chips refer to, for example, an inner chip for drilling a well in the vertical direction and a gage pad for drilling the well in the radial direction of the well. The present invention further relates to an oil-drilling tricone bit having the insert chips as described above.
2. Description of the Background Art
A tool called a tricone bit is used for drilling an oil well for example. The tricone bit is used for drilling subterranean rocks and thus the tricone bit generally has, as its cutting edges, insert chips made of WCxe2x80x94Co-based cemented carbide with good abrasion resistance.
Insert chips for the tricone bit are generally classified into two types, i.e., an inner chip for vertical drilling of an oil well and a gage pad for drilling the oil well in the radial direction. An inner chip and a gage pad are schematically shown in FIGS. 9 and 10 respectively.
In recent years, oil wells are drilled at increasingly greater depths and accordingly rocks themselves at such great depths are hard to drill. Because of this, insert chips that are cutting edges of the tricone bit wear at earlier stages or some fragments of insert chips are broken (chipped) off from the insert chips (chipping of insert chips). A resultant problem is a shortened lifetime of the tricone bit. Moreover, a considerably costly work is necessary for lifting the tricone bit which reaches the end of its lifetime at several thousand meters below ground and for replacing the tricone bit with new one in order to proceed with drilling. Then, there is a need for further increase in the lifetime of insert chips.
Under the situation as described above, both of abrasion resistance and resistance to chipping (hereinafter chipping resistance) of insert chips must be improved. In general, cemented carbide has a higher hardness when it contains a smaller amount of Co and thus has an improved abrasion resistance, while the smaller amount of Co results in a higher brittleness of the cemented carbide which deteriorates the chipping resistance. In other words, the abrasion resistance and chipping resistance are not compatible with each other
Various arts have been well known that concern the need for increase in the lifetime of insert chips as detailed below.
Japanese Patent Laying-Open No. 5-209488 discloses a rock-drilling button having an xcex7 (eta)-phase core exposed at the top and a surface region having a high Co content that is formed to enclose the xcex7-phase core, produced by adjusting sintering conditions of cemented carbide. According to the art disclosed, abrasion is alleviated since the exposed xcex7-phase touches rocks from the start of drilling. On the other hand, the high Co content in the surface region enhances the chipping resistance. A problem with this art is that the cemented carbide composition containing the xcex7-phase which is an embrittlement phase is requisite. In general, the xcex7-phase included in the cemented carbide could be an origin from which the metal is likely to chip off, resulting in deterioration of reliability.
Japanese Patent Laying-Open No. 7-150878 discloses an art of improving peeling resistance of the outermost polycrystalline diamond layer of an insert. This insert has a substrate made of sintered tungsten carbide, the outermost surface of a cutting edge of the insert is covered with the polycrystalline diamond layer, and an intermediate layer is provided between the substrate of sintered tungsten carbide and the polycrystalline diamond layer. The intermediate layer is a composite-material layer made of sintered tungsten carbide and polycrystalline diamond. However, a problem with this art is that the polycrystalline diamond itself has a low toughness which causes any crack in the outermost polycrystalline diamond layer and consequently the crack becomes an origin from which the insert breaks off.
Japanese Patent Laying-Open No. 11-12090 discloses a similar art according to which CVD (chemical vapor deposition) is used for coating a surface of a drill bit made of cemented carbide with diamond. However, the diamond and cemented carbide are different in thermal expansion coefficient which could cause a problem of peeling.
Japanese Patent Laying-Open No. 8-170482 proposes a drill bit having a hardness gradient. Specifically, the lowest hardness of a substrate of an insert chip of the drill bit increases gradually toward the leading end of the insert chip. It is noted that the tricone bit includes cone sections in which respective insert chips are fit and a body holding the cone sections, and not only the cone sections rotate but also the body itself rotates for drilling. Accordingly, not only tips of cutting edges of the insert chips but also sides of the cutting edges contribute to drilling. If the art disclosed in Japanese Patent Laying-Open No. 8-170482 is applied to insert chips of a tricone bit, the cutting edge has its side where cemented carbide of low hardness, i.e., low abrasion resistance, is exposed, since the insert chip is formed of a stack including cemented carbide materials of different compositions bonded to each other. A resultant problem is that the side of the cutting edge predominantly wears to shorten the lifetime.
Japanese National Patent Publication No. 10-511432 proposes an insert chip having a substrate of a cemented carbide and a cutting edge coated with one coating layer of a cemented carbide different from that of the substrate. Specifically, the coating layer of the cemented carbide has a lower Co content than that of the substrate for improving the abrasion resistance of the insert chip and satisfying the chipping resistance requirement by the substrate. It is known that decrease of Co content of cemented carbide decreases thermal expansion coefficient thereof. Then, if the difference in Co content between the substrate and the coating layer with which the substrate is coated is excessively large, a resultant problem is that the coating cemented carbide layer is peeled off or any crack occurs, for example. Then, according to this art, the coating cemented carbide layer cannot have its Co content greatly different from that of the substrate and thus the improvement of abrasion resistance is limited.
As discussed above, various studies have been conducted on insert chips for drilling and drill bits. However, there is still a need for an insert chip, especially an insert chip of an oil-drilling tricone bit, that is suitable for drilling rocks which are at greater depths and accordingly difficult to drill and that has both of abrasion resistance and chipping resistance.
One object of the present invention is to provide an insert chip of an oil-drilling tricone bit and a method of manufacturing the insert chip, the insert chip having both of abrasion resistance and chipping resistance. It is also an object of the present invention to provide an oil-drilling tricone bit suitable for drilling rocks which are at greater depths and thus hard to drill.
An insert chip of an oil-drilling tricone bit according to the present invention includes, in order to achieve the above-described objects, an insert-chip substrate made of a cemented carbide of a first composition, the substrate including a cylindrical body and a cutting edge for drilling, and includes a cemented carbide coating layer formed of at least two stacked coating layers made of a cemented carbide of a composition different from the first composition, the cemented carbide coating layer covering at least 80% of the surface area of the cutting edge of the insert-chip substrate. The coating layers each have a thickness, at a tip portion of the cutting edge, ranging from 0.1 mm to 2.5 mm and, the total thickness of the cemented carbide coating layer ranges from 1 mm to 5 mm. The coating layers include an outermost cemented carbide layer and at least one coating layer besides the outermost cemented carbide layer and the outermost cemented carbide layer has a hardness higher than that of that at least one coating layer and of the insert-chip substrate. By the above-described structure, it is possible to achieve a high abrasion resistance by the outermost cemented carbide coating layer and improve the chipping resistance by other coating layer(s) and the insert-chip substrate. Moreover, intermediate layer(s) lessens thermal stress, which accordingly prevents peeling and crack of the coating layers.
Preferably, the cemented carbide coating layer covers the whole of the cutting edge. Thus, the abrasion resistance can further be improved.
Preferably, those at least two stacked coating layers include, besides the outermost cemented carbide layer, an anti-chipping layer of a composition with a higher Co content than that of the outermost cemented carbide layer or with a larger WC particle size than that of the outermost cemented carbide layer. More preferably, the anti-chipping layer has a composition with a higher Co content than that of the insert-chip substrate. The chipping resistance can thus be improved. The anti-chipping layer which is one of the coating layers is accordingly thin, 2.5 mm or less in thickness. Therefore, resistance to plastic deformation is superior to the deformation resistance obtained by using a cemented carbide with a high Co content for the insert-chip substrate.
Preferably, the anti-chipping layer contains Co particles including special Co particles each elongated in the radial direction of the insert chip in a vertical cross sectional structure of the insert chip and each having a ratio ranging from 3 to 100 that is the length in the radial direction of the insert chip/the length in the axial direction of the insert chip, and the special Co particles constitute at least 5% by volume of the Co particles contained in the anti-chipping layer. Thus, it is possible to prevent any crack from opening and further running and accordingly improve the anti-chipping property.
Preferably, the outermost cemented carbide layer contains WC of an average particle size of at most 1 xcexcm. Thus, it is possible to prevent WC particles from dropping off and accordingly increase the surface area of one WC particle, which improves adhesion between WC and Co.
Preferably, the outermost cemented carbide layer includes compressive residual stress. Thus, it is possible to prevent thermal crack and accordingly improve chipping resistance.
Preferably, the outermost cemented carbide layer includes compressive residual stress ranging from 0.05 GPa to 0.80 GPa. Thus, it is possible to prevent thermal crack from occurring without breakage of the layer itself.
Preferably, only the outermost cemented carbide layer among the coating layers contains diamond particles of a particle size ranging from 10 xcexcm to 100 xcexcm and the diamond particles constitute 5% to 40% by volume of the outermost cemented carbide layer. Thus, it is possible to enhance abrasion resistance relative to cemented carbide while diamond particles are unlikely to drop off.
Preferably, the diamond particles are each covered with at least one of refractory metal and ceramic of at most 1 xcexcm in thickness. Thus, it is possible to improve the wetting property between the diamond particles and cemented carbide and accordingly improve the adhesion property therebetween.
Preferably, the outermost cemented carbide layer has a micro Vickers hardness of at least 15 GPa. Thus, it is possible to improve the abrasion resistance by the outermost cemented carbide layer with the chipping resistance maintained by those layers under the outermost layer.
An oil-drilling tricone bit according to the present invention includes, as its cutting edge, in order to achieve the above-described objects, any insert chip of an oil-drilling tricone bit as detailed above. By xe2x80x9cinsert chip of an oil-drilling tricone bitxe2x80x9d provided as a cutting edge, the high abrasion resistance is achieved by the outermost cemented carbide layer and the chipping resistance is improved by remaining coating layer(s) and the insert-chip substrate. Then, the oil-drilling tricone bit has superior drilling performance for rocks at greater depths and thus hard to drill while having a long lifetime.
A method of manufacturing an insert chip of an oil-drilling tricone bit includes, in order to achieve the above-described objects, an inserting step of inserting an insert-chip substrate into a die, a stacking step of stacking, on the insert-chip substrate, cemented carbide powder to form a coating layer having a desired thickness after being sintered, and a sintering step of performing electrical pressure sintering, by using a punch inserted into the die, the punch having a depressed end which matches in shape a protruded cutting edge of the insert chip, applying a pressure ranging from 20 MPa to 50 MPa, and controlling the temperature of the punch within a temperature range from 1500xc2x0 C. to 1800xc2x0 C. By this method, it is possible to manufacture an insert chip of an oil-drilling tricone bit having its cemented carbide layer without gross porosity or cavity, without seepage of Co, and without mold breakage.
Preferably, in the sintering step, sintering is performed for a period ranging from 5 minutes to 20 minutes. By this method, it is possible to manufacture an insert chip of an oil-drilling tricone bit having denser cemented carbide without abnormal growth of WC particles.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.